Microbial pathogenesis is influenced by a human innate immune response that includes the synthesis and release of antimicrobial peptides. This proposal seeks continuing support for our studies of the mechanism of action of a family of antimicrobial peptides known as cathelicidins. Our approach of employing modem immunological techniques including in vitro systems, mouse genetics and clinical analysis has advanced understanding of innate immunity and uncovered several situations whereby cathelicidin expression influences susceptibility to infection and results in human disease. Combined with this, a simultaneous molecular microbiology approach has revealed multiple strategies where microbes obtain cathelicidin resistance to initiate infection. Together, these studies support our central hypothesis that cathelicidins are critical to mammalian immune defense and influence the virulence potential of microbial pathogens. Important questions relevant to this hypothesis remain unanswered including;identification of the forms of cathelicidin released during skin infection, determination of the basis for cathelicidin resistance in the major skin pathogen, and understanding how the host regulates the expression of cathelicidin in response to infection. To address these critical issues this proposal has the following specific aims: Specific Aim 1: Investigate the function of distinct structural domains of cathelicidins by identification of peptides released during infection of the skin and analysis of the structure and function of these as antimicrobials and host immunomodulators. Specific Aim 2: Determine mechanism and impact of cathelicidin resistance by understanding the molecular basis of cathelicidin resistance in Staphylococcus aureus through analysis of cathelicidin-sensitive strains in a random transposon mutant library and correlation with disease virulence in mice. Specific Aim 3: Discover mechanisms of cathelicidin host response to microbial pathogenesis by screening human pathogenic and nonpathogenic organisms for cathelicidin stimulatory potential and using a molecular genetic approach to discovery of microbial cathelicidin-stimulating or suppressing factors. Addressing the aims of this proposal will continue to provide insight into the pathogenesis of human infectious disease and offer additional novel alternatives to standard antimicrobial therapy.